[Google](http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Comments#Comments), as well as [PEP257](http://legacy.python.org/dev/peps/pep-0257/) require docstrings to have a one liner that fits on one line.  If it needs more, then there should be one line, followed by a blank line, followed by the longer summary.  This allows tools like pydoc to extract one line summaries from all docstrings.  

     """Returns logout url if user is logged in; otherwise returns None."""       

fits in one line.